1. Field of the Invention
The invention is generally related to the area of optical communications. In particular, the present invention is related to optical wavelength multiplexing or add/drop devices with high reflection channel isolation and the method for making the same in compact size.
2. The Background of Related Art
The future communication networks demand ever increasing bandwidths and flexibility to different communication protocols. Fiber optic networks are becoming increasingly popular for data transmission due to their high speed and high capacity capabilities. Wavelength division multiplexing (WDM) is an exemplary technology that puts data from different sources together on an optical fiber with each signal carried at the same time on its own separate light wavelength. Using the WDM system, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed into a light stream transmitted on a single optical fiber. To take the benefits and advantages offered by the WDM system, there require many sophisticated optical network elements.
Optical add/drop and multiplexer/demultiplexer devices are those elements often used in optical systems and networks. For example, an exchanging of data signals involves the exchanging of matching wavelengths from two different sources within an optical network. In other words, an add/drop device can be advantageously used for the multi-channel signal for dropping a wavelength while simultaneously adding a channel with a matching wavelength at the same network node. Likewise, for transmission through a single fiber, a plurality of channel signals are combined via a multiplexer to be a multiplexed signal that eventually separated or demultiplexed via a demultiplexer.
A fundamental element in add/drop devices and multiplexer/demultiplexer is what is called a three-port device. As the name suggests, a three-port device has three ports, each for a multi-channel signal, a dropped or added signal or a multi-channel signal without the dropped or added signal. FIG. 1A shows a typical design of a three-port add/drop device 100. The optical device 100 includes a common (C) port 102, a reflection (R) port 104, and a transmission (T) port 106. When the device 100 is used as a multiplexer (i.e., to add a signal at a selected wavelength λK to other signals at wavelengths other than the selected wavelength λK), the T-port 106 receives a light beam at the selected wavelength λK that is to be multiplexed into a group of beams at wavelengths λ1, λ2, . . . λN excluding the selected wavelength λK coupled in from the C-port 102. The R-port 104 subsequently produces a multiplexed signal including all wavelengths λ1, λ2, . . . λK, . . . λN.
Likewise, when the optical device 100 is used to demultiplex signals, the C-port 102 receives a group of signals with wavelengths λ1, λ2, . . . . λK, . . . λN. The T-port 106 produces a signal with the selected wavelength λK while the R-port 104 subsequently produces a group of signals including all wavelengths λ1, λ2, . . . λN except for the selected wavelength λx. In general, the optical paths towards a R-port and a T-port are respectively referred to as R-channel and T-channel.
FIG. 1B shows an exemplary internal configuration 110 of the optical device 100 of FIG. 1A. As shown in FIG. 1B, there is a first GRIN lens 112, an optical filter 114 (e.g., a multi-layer thin film filter) and a second GRIN lens 116. In general, a dual-fiber pigtail is provided in a holder 118 (e.g., a dual-fiber pigtail collimator) and coupled to or positioned towards the first GRIN lens 112, and a single-fiber pigtail is provided in a second holder 120 and coupled to or positioned towards the second GRIN lens 116. Essentially the two GRIN lenses 112 and 116 accomplish the collimating means for coupling an optical signal with multi channels or wavelengths in and out of the C port 102, the R port 104, or the T port 106. In general, the three-port device 100 is known to have a very low coupling loss from the C-port to both the R-port and the T-port for use as a demultiplexing device, or vise versa as a multiplexing device.
As a result, the three-port device 100 is often used to assemble a four-port thin film interference filter based optical wavelength add/drop device. FIG. 2A shows a four-port add/drop device 200 and respective functions of the four ports labeled, respectively. An incoming network fiber provides a light beam including wavelength division multiplexed (WDM) optical signals, for example, with or at wavelengths λ1, λ2, . . . , λk, . . . λn, to an input port 202 of the device 200. A predetermined signal channel which is carried by a wavelength λk is dropped, for example, to a local site for use through a drop port 204. At the same time, a new signal at a wavelength λk′ that is identical or substantially similar to the dropped wavelength λk is coupled to an add port 208. A newly combined or multiplexed signal including wavelengths λ1, λ2, . . . , λk′ . . . λn is out from an express port 206.
Traditionally, the four port device 200 is made by cascading two three-port devices. FIG. 2B shows an internal configuration 210 employing two three-port devices 212 and 214, such as the one 100 of FIG. 1B. Essentially, the three-port devices 212 and 214 are cascaded to form a four port add/drop device.
In general, the optical filters (e.g., thin film filter or TFF) can achieve nearly 100% reflection for the R-channel. For instance, TFF with a reflection index of 99.99% to 99.999% can be routinely achieved and commercially available. This is equivalent to 40 dB to 50 dB isolation for the T-channel from the R-channel. In other words, the mount of residual signal of the R channel transmitted through the TFF and mixed into the T-channel signal is −40 dB to −50 dB of the R-channel signal. The signal exiting the T-port of the 3-port device contains basically the pure T-channel signal, with the R-channel signal residual of −40 dB to −50 dB of its original signal intensity.
However, the optical filters by the state-of-art TFF deposition technique can only achieve 97.5% to 99% transmission for the T-channel signal. This is equivalent to 15 dB to 20 dB isolation for the R-channel signal from the T-channel signal. The signal exiting the R-port of the 3-port device contains not only R-channel signal, but also some residual of the T-channel signal that can be of −15 dB to −20 dB of its original signal intensity. As a result, the residual of the T-channel signal would interfere with the R-channel signal, especially when a new signal is added into the transmission fiber that is using the same T-channel optical carrier signal, leading to complexity and difficulty when processing the R-channel signal.
To increase the isolation for the R-channel signal from the T-channel signal, several methods have been used for removing the T-channel signal residual in the R-channel signal. One approach that is commonly adopted is to use a pair of conjugate filters with the corresponding spectral characteristics are reciprocal to each other. FIG. 3 shows one of the practical implementation of the above idea. The filters 302 and 304 are deposited with multiplayer coatings to allow transmitting only wavelength λ1 and λ2, respectively. When a light beam including signals at various wavelengths including λ1 and λ2 is coupled by the collimator 306 to the filter 302, a substantial portion of the signal at λ1 passes the filter 302, and at the same time, nearly all the signal λ2 and the residual portion of λ1 are reflected to the filter 304. The reflected light beam then impinges upon the filter 304 that transmits only a signal at λ2. As a result, the signal at λ2 is pure and is isolated from possible interference. Although this approach increases the R-channel isolation, this approach demands stringent assembling tolerance and technical challenge. In addition, the high isolation device build in such way suffers high cost and bulky size.
US Patent Application publication 2003/0228101 discloses the use of multiple filters in a compact package form. FIG. 4 duplicates FIG. 1 of US Patent Application publication 2003/0228101 in which a filter is attached to an end surface 34 of a second fiber or a rear face 40 of a GRIN lens 4. In operation, an incoming light signal includes two wavelength λ1, λ2 and travels from the first fiber 30 through the GRIN lens 4, then reaches the first filter 5. The first filter 5 is designed to just pass a light with a wavelength of λ1. The light of wavelength λ1 goes through the first filter 5 to reach another device, for example, a single fiber collimator. The light signal reflected by the first filter 5 then goes back through the GRIN lens 4 and reaches the second filter 6. The second filter 6 is designed to just pass a light only with a wavelength of λ2. The light of wavelength λ2 goes through the second filter 6 and travels along the second fiber 32 to another device for further processing.
In other words, the two filters 5 and 6 are conjugate. The second or conjugate filter 6 is deposited either on the end facet 40 of the R-channel fiber or the surface 34 of the fiber 32. The film filter deposited there has to be thin enough so that it can be fit into an air gap between the dual fiber pigtail 3 and the R-channel lens 4. The gap thickness is typically in the range of few tens micrometers to a few hundreds micrometers. This approach has the advantage of enjoying the same compact size and reliability as the standard three-port devices. However, the performance is not as desirable as the free-space approach shown in FIG. 3.
Generally, a fiber end facet is made to have a slanted angle θ for the purpose of reducing back reflection. Typically, the slanted angle θ=8° is widely adopted, but the angle is not always confined to 8°. With such slanted angle, the returned light imaged onto the R-channel fiber is no longer aligned with the mechanical axis of the R-channel fiber, resulting in an offset angle of φ=3.65°, when the light wavelength is near 1.5 μm and when the fiber core index is 1.45. FIG. 5 shows the relationships of the slanted angle θ, the offset angle φ, and the fiber (mechanical) axis of a fiber 400 with a slanted facet 402.
Because of the slanted facet, the light incident angle on the filter is 8°+3.65°≈12°. As a result, the filter performance is severely degraded when the light incident is at such high range, as it is known in the art that the filter performance depends on the light incident angle. FIG. 6 is a testing result, clearly showing the deadband increase as the result of incident angle increasing from 0° to 10°. It is also clear that the deadband increases non-linearly with the incident angle: when the incident angle is very small, the deadband is the same; while after 4°, the deadband increases dramatically with the further increase of the incident angle. For many fiber optic telecommunication applications, such as the fiber to the home (FTTH) applications, it requires that a deadband of a filter (e.g., thin film filter or TFF) is no more than 35-50 nm with isolation better than 40 dB, wherein a deadband is referred to a region between a stopband and a passband of the spectrum of the filter. Practically, it is difficult to meet the requirements when the light incident angle is at ˜12°.
Accordingly, there is a great need for techniques for providing high isolation from the T-channel channel such that the errors or residuals to the R-channel are minimized. The devices so designed are amenable to small footprint, broad operating wavelength range, enhanced impact performance, lower cost, and easier manufacturing process.